1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device having a memory array in which a plurality of memory cells each formed by connecting one end of a variable resistive element for storing information according to a change in electric resistance caused by application of a voltage and a drain of a selection transistor to each other on a semiconductor substrate are arranged in the row direction and the column direction, and a control method thereof. More particularly, the present invention relates to a method of applying a voltage to a memory cell at the time of programming or erasing.
2. Description of the Related Art
In recent years, it is getting easier and easier to obtain information anytime and anywhere and to freely carry the information. Because of spread of mobile apparatuses typified by a portable telephone and a PDA (Personal Digital Assistant), the user can access various information anywhere and anytime. However, performances of the mobile apparatus such as the life of a battery of the mobile apparatus and access speed to information are not sufficiently high yet and there are unlimited demands for improvements in the performances. Particularly, the life of a battery is one of main performances determining the usability of a mobile apparatus, so that reduction in power consumption is strongly requested for the components of a mobile apparatus.
As one of key devices, a nonvolatile semiconductor memory is becoming more and more important. In a mobile apparatus, in an active operation state, power consumption of a logic circuit for executing a logic function is dominant but, in a standby state, power consumption of a memory device is dominant. The power consumption in the standby state is becoming more important as the drive time by a battery of a mobile apparatus increases. By using the nonvolatile semiconductor memory, it becomes unnecessary to supply power to the nonvolatile semiconductor memory in the standby state, so that the power consumption can be reduced to the limit.
Nonvolatile semiconductor memories include a flash memory, an FeRAM (Ferroelectric Random Access Memory) and the like, and many of them are already practically used. Those nonvolatile semiconductor memories have tradeoffs among high speed, rewrite resistance, power consumption and the like. Research and development for an ideal nonvolatile semiconductor memory satisfying all of required specifications are being conducted. Some nonvolatile semiconductor memories using new materials have been already proposed and an RRAM (Resistance Random Access Memory) is one of promising nonvolatile semiconductor memories. Since the RRAM has high potentials of high speed, large capacity, low power consumption and the like, expectations are placed on the future potential of the RRAM.
Zhuang, H. H., et al., “Novel Colossal Magnetoresistive Thin Film Nonvolatile Resistance Random Access Memory (RRAM)”, IEDM, Paper No. 7.5, December, 2002 specifically describes an RRAM in which by applying a voltage pulse to an oxide material containing manganese having a perovskite type crystal structure showing CMR (colossal magnetoresistance) and HTSC (high temperature super conductivity) such as Pr1-xCaXMnO3 (0<x<1, hereinafter, abbreviated as “PCMO”), the resistance value changes.
As a concrete characteristic, FIG. 13, in which the vertical axis indicates the resistance value and the horizontal axis indicates the number of pulse application times, shows a change in the resistance value when a pulse of 100 nanoseconds of ±5 V is applied to the PCMO having a thickness of 100 nm. By application of pulses, the resistance value changes between 1 kΩ and 1 MΩ and a large change of three digits in the resistance value occurs reversibly 100 times or more. FIG. 14, in which the vertical axis indicates the resistance value and the horizontal axis indicates the number of application times of a pulse of 4 V and 5 nanoseconds, shows that the resistance value of the variable resistive element changes in an analog manner in accordance with the number of pulse application times. Not only two states of a low resistance state (for example, 1 kΩ or less) and a high resistance state (100 kΩ or more) but also an arbitrary resistance state can be set. Consequently, for example, by dividing the resistance value into four states as shown in FIG. 17 between 10 kΩ and 1 MΩ, the resistance value can take multiple values and the bit cost can be reduced. It is expected that, by using such a variable resistance element for a memory carrier, an ideal high-speed, large-capacity nonvolatile semiconductor memory can be realized.
However, according to Hsu, S. T. et al., “Charge Transport Property in Non-Volatile Resistor Random Access Memory (RRAM)”, Non-Volatile Semiconductor Memory Workshop 2003, pp. 97–98, February, 2003, as shown in FIGS. 15 and 16, the electric conductivity characteristic of the variable resistive element is almost linear when a current (I)-voltage (V) characteristic in each of the high resistance state and the low resistance state is plotted in a graph having the vertical axis of logarithm (LogI) of current and the horizontal axis of the square root (√V) of voltage. From the result, it can be said that the electric conductivity in each of the high and low resistance states displays nonlinear electric conductivity of the Poole-Frenkele type.
As the Poole-Frenkele type electric conductivity in the variable resistive element, the relation between the current value I and the voltage V is expressed by I∝Exp (√V). It indicates that the current-voltage characteristic has very high nonlinearity and a small voltage change causes a large change in the current amount. Consequently, if there are variations in a program threshold voltage which changes from the low resistance state (hereinafter, described as RL) of a variable resistive element to the high resistance state (hereinafter, described as RH) or an erase threshold voltage which changes from the RH to the RL, when the same voltage is applied to the variable resistive element to program or erase data, a very large variation occurs in the amount of current flowing in the variable resistive element, and current consumed in programming or erasing increases.